No, not every combination will work :P And flashtool is used to repair phone, not to flash custom ROMs. There are some ROMs working on stock kernel (which U have now) - mainly based on stock .431 or .587 software. U can flash them using recovery (which U have now). To be honest I don't know what are the differences between wt19i and wt19a. But as a experiment (lol) I flashed baseband from SK17a into my SK17i and everything was ok. That's why I told U to create a ftf package, which will allow U to flash stock ROM if something went wrong.
So, for now:
If U would like install any ICS ROM (based on original Sony .431 or .587 compilation - You can try it right away.
If U would like to flash CyanogenMod ROMs (CM7.2 [android 2.x GingerBread], CM9 [android 4.0.x IceCreamSandwich], CM10 [android 4.1 JellyBean], CM10.1 [android 4.2 JellyBean], CM10.2 [android 4.3 JellyBean] ) and other based on it, U will have to unlock bootloader at the first place and then flash kernel suitable for Your device (LWW is coded as coconut) and then flash ROM.
Xperia X8 black +
Xperia mini pro black +
nAa 188.8.131.52 kernel +
JellyVanilla 7.1 +
16GB Sandisk Ultra class10 microSDHC +
Panasonic RP-HJE120 ergofit